import java.util.Scanner;

import java.lang.*;

/**

* Created by Lemon on 2017-08-18.

*/

public class solution3 {

public static void main(String[] args) {

Scanner in = new Scanner(System.in);

String str = in.nextLine();

System.out.println(getLongestStr(str));

}

public static int getLongestStr(String str1) {

int[] hashTable = new int[256];

for (int p = 0; p < 256; p++) {

hashTable[p] = 0;

}

char[] str = str1.toCharArray();

int start = 0;

int mstart = 0;

int mlen = 0;

int idx = 0;

int len = 0;

while (idx != str.length) {

if (hashTable[str[idx]] == 1) {

if (len > mlen) {

mstart = start;

System.out.println(str[idx]);

mlen = len;

}

while (str[start] != str[idx]) {

hashTable[str[start]] = 0;

start++;

len--;

}

start++;

} else {

hashTable[str[idx]] = 1;

len++;

}

idx++;

}

if (len > mlen) {

mlen = len;

mstart = start;

}

return mlen;

}

}